It is not uncommon for a user seeking entry to a locked room or dwelling, mind preoccupied with various matters and/or hands holding or maneuvering a multiplicity of bags or packages and the like, to unlock the passage door with the appropriate key and then, in gaining entry to the formerly restricted area, to leave the key in the lock keyhole. Sometimes the user knowingly leaves the key in the lock with the intention of returning to the door to recover the key after ridding himself of packages, bags and outer clothing or the like. In other instances the user may simply forget or not realize his failure to remove the key from the lock before the door is manually closed or is automatically caused to shut by a commonly spring-loaded design. In numerous such instances, the key remains in the door lock for an extended period of time, and is often not missed or recovered until the next time that the user seeks to leave the locked area. All too often, by that time the key has been removed by unknown persons who may intend to use the key for unconsented to, unrestricted and possibly unlawful access into the locked space.
Prior art devices have recognized this problem and have accordingly sought to alert the user to his omission with an alarm. These prior art arrangements, however, have not been entirely satisfactory.
U.S. Pat. No. 901,983 provides an example of an alarm device which is activated by insertion of a key or tool into a keyhole of a lock. In this alarm device a displaceable pin is inserted into the keyhold from one side of the lock. When a key or other object is inserted into the keyhole it pushes the pin out of the keyhole and into abutment with an electrical contact to close an alarm circuit and sound a warning alarm. The alarm sounds as long as the key remains in the lock or tampering with the lock continues.
Alarm devices of this type suffer the annoying disadvantage that the alarm continuously sounds for the entire period that a key is inserted in the lock. In addition, the alarm device disclosed in U.S. Pat. No. 901,983 works only in those locks having two keyholes, respectively accessible from both sides of the door in which the lock is installed.
A need therefore exists for a warning device which will sense the presence of a key in a lock but which will not produce warning alarm unless the key is inadvertently left in the lock. Additionally, there exists a further need for a warning device which can be used in association with virtually any type of commercially available lock so long as the lock is operated by a key.